1. Field of the Invention
This invention relates to an image sensing device having a white compression circuit.
2. Description of the Related Art
The dynamic range of a video signal obtained by an image sensing device is generally larger by a plurality of times than that of an ordinary standard TV signal. Therefore, for the purpose of securing an adequate dynamic range or S/N ratio, it has been practiced to carry out a white compression (knee) process within an analog or digital signal processing system. Various methods for attaining this purpose have been known.
The conventional white compression process has been generally carried out at a fixed point after a white balance adjusting process. However, for example, it is conceivable to carry out the white balance adjustment by varying the reference voltages of the two ends of the ladder resistance of an A/D converter. In such a case, the white compression process must be carried out before the white balance adjustment process (in the case of R, G and B signals, for example) for the purpose of matching the input signal with the inside of the dynamic range of the A/D converter. Then, if the operating point of the white compression process is set beforehand at a level within a range desired for adequate white balance adjustment, the white compression characteristic becomes non-linear at a high luminance part within this range. The non-linear characteristic then results in a white compression process effected only for the R or B signal among other input signals according to the levels of input color signals. In such a case, therefore, the white balance adjustment cannot be accurately accomplished.